1. Field of the invention
The present invention relates to a thin film transistor (TFT), more particularly, to a TFT having a lightly doped drain (LDD) structure.
2. Discussion of the Background
Generally, a TFT comprises a semiconductor layer, a gate electrode and source/drain electrodes. The semiconductor layer comprises source/drain regions and a channel region interposed between the source/drain regions, and it may be formed of polycrystalline or amorphous silicon. Polycrystalline silicon is usually used since it has higher electron mobility than amorphous silicon.
However, a polycrystalline silicon TFT has a higher off current than an amorphous silicon TFT. Forming an LDD region between source/drain regions and the channel region of the polycrystalline silicon TFT may decrease its off current because the LDD region may prevent a rapid increase of electric field between the channel region and the source/drain regions. The LDD region may also suppress hot carrier effect (HCE) generated as the channel region's length decreases. Therefore, the LDD region may improve channel reliability.
In the LDD region, however, a profile of ions doped may have a considerable effect on a TFT's characteristics, including channel reliability.